Analog fiber optic links have become an important tool in modern communications. Such fiber optic links are used in numerous communications applications such as antenna-remoting, delay lines, signal-processing, and radio-over-fiber applications, just to name a few. See J. E. Roman, et al., “Fiber-optic remoting of an ultrahigh dynamic range radar,” IEEE Trans. Microwave Theory Tech. 46, 2317-2323 (1998); C. Chang, et al., “Fiber optic delay line devices for RF signal processing,” Electron. Lett. 13, 678-680 (1977); S. D. White, “Implementation of a photonic automatic gain control system for correcting gain variations in the Green Bank Telescope fiber optic system,” Review of Scientific Instruments, vol. 71, pp. 3196-3199 (2000); A. L. Campillo, et al., “Phase performance of an eight-channel wavelength-division-multiplexed analog-delay line,” IEEE J. Lightw. Technol., vol. 22, pp. 440-447 (2004); and E. E. Funk, et al., “High dynamic range, long haul (>100 km) radio over fiber,” in Microwave Photonics, Boca Raton: CRC Press, 2007, ch. 6, pp. 185-212.
Many of these links use Mach-Zehnder intensity modulators (MZM) to impose an RF signal on a single optical carrier. The RF optical signal modulated by the MZM is then amplified and passed down the optical fiber link, where it is recovered by direct detection at a photodiode. The MZM is quadrature-biased and its use adds no even-order distortion to the RF signal.
Various suppressed carrier techniques have been demonstrated to improve the performance of these analog optical fiber links, including filtering of the input optical signal, using a null-biased Mach-Zehnder modulator with a local oscillator, and using a low-biased Mach-Zehnder modulator. See M. J. LaGasse, et al., “Optical carrier filtering for high dynamic range fibre optic links,” Electronics Letters, vol. 30, pp. 2157-2158 (1994); X. B. Xie, et al., “Suppressed-carrier large-dynamic-range heterodyned microwave fiber-optic link,” IEEE International Topical Meeting on Microwave Photonics, Ogunquit, Me. (2004); M. L. Farwell, et al., “Increased linear dynamic range by low biasing the Mach-Zehnder modulator,” IEEE Photonics Technol. Lett., vol. 5, pp. 779-782 (1993); A. Karim et al., “Noise Figure Reduction in Externally Modulated Analog Fiber-Optic Links,” IEEE Photonics Technol. Lett., vol. 19, pp. 312-314 (2007); and X. J. Meng et al., “Microwave Photonic Link with Carrier Suppression for Increased Dynamic Range,” Fiber and Integrated Optics, vol. 25, pp. 161-174 (2006).
In addition, analog metrics such as RF noise figure, spur free dynamic range (SFDR) and compression dynamic range (CDR) improve with increased photocurrent, and so amplification of the signal often is desirable to maximize the optical power reaching the photodiode. Unfortunately, the nonlinear Stimulated Brillouin Scattering (SBS) effect limits the amount of optical power that can enter the fiber. One method to increase the SBS threshold is to suppress the optical carrier by low-biasing the MZM. See M. M. Sisto, et al, “Carrier-to-noise ratio optimization by modulator bias control in radio-over-fiber links,” IEEE Photonics Technol. Lett. 18, 1840-1842 (2006); L. T. Nichols, et al., “Optimizing the ultrawide-band photonic link,” IEEE Trans. Microwave Theory Tech. 45, 1384-1389 (1997); M. T. Abuelma' Atti, “Large signal analysis of the Mach-Zehnder modulator with variable bias,” Proc. Natl. Sci. Counc. ROC(A) 25, 254-258 (2001). The use of a high power laser with a low-biased MZM has been shown not only to improve the SBS threshold, but also to improve the RF noise figure of an analog link. See A. Karim et al., supra.
However, the previous works have not considered the benefits of post-modulator amplification for low-biased MZM links. Recent work has shown that a low-biased MZM followed by optical amplification increases the RF gain of an analog link beyond simply increasing the optical power at the photodiode. M. M. Sisto, et al., “Erbium amplifier dynamics in wireless analog optical links with modulator bias optimization,” IEEE Photonics Technol. Lett. 19, 408-410 (2007); V. J. Urick, et al., “Analysis of an analog fiber-optic link employing a low-biased Mach-Zehnder modulator followed by an Erbium-doped fiber amplifier,” IEEE J. Lightwave Technol. 27, 2013-2019 (2009). However, although the various low-biased MZM techniques have been shown to improve the RF performance of fiber-optic analog links, they have a serious disadvantage when compared to quadrature-biased MZM links because the even-order harmonics become quite large, and thus the use of low-biased MZMs is limited to single-octave applications.
One approach to solving this problem cancels the even-order harmonics by low-biasing two wavelengths on either side of the null of the bias curve with a single output MZM. See U.S. Pat. No. 7,079,780, to Rollins, “Linearized optical link using a single Mach-Zehnder modulator and two optical carriers.” However this method does not demonstrate an improvement in the RF metrics from low-biasing the wavelengths.
Another technique uses two separate MZMs in order to cancel the unwanted harmonics. W. K. Burns, et al. “Multi-octave operation of low-biased modulators by balanced detection,” IEEE Photonics Technol. Lett. 8, 130-132 (1996). However, this method requires the RF signal be split to two MZMs, which results in higher loss for the RF signal.
Another method inputs two optical carriers at different wavelengths through a single MZM. The use of two optical carriers has been shown to linearize unwanted harmonics in a fiber link. E. Ackerman, “Broad-band linearization of a Mach-Zehnder electrooptic modulator,” IEEE Trans. Microwave Theory Tech. 47, 2271-2279 (1999). Unfortunately, the linearization process reduces the RF power of the fundamental while reducing the harmonic, and requires over 200 nm of separation in the wavelengths of the two optical carriers, and thus this method has limited usefulness and is not suitable for many applications.